温度梯度下高压直流绝缘介电性能的长期演变规律与寿命评估

Recently, with a rapid development of HVDC cables, more and more HVDC cables put into research and operation in home and abroad. However few studies have been paid attention to the long-term ageing performance of HVDC cable. At present, the concept that space charge plays the decisive role during the operation and ageing of HVDC cables has been accepted by many researchers in home and abroad. As a result, the new materials such as the representative nano-composites have been developed in order to inhibit space charge effects in DC cable insulation. But the long-term performances and life evaluations of the novel nano-composites still need to be further gone into. Moreover, previous studies showed that temperature gradient effect during the operation of DC cables will seriously distorted the field near the low temperature side of insulation. However the study on failure mechanism of longer-term ageing for DC insulation under temperature gradient is still lacking. In this project, through the researches on the development of various macro- and micro- properties of HVDC cable insulations during the process of ageing under DC stress with the cooperation of temperature gradient, the ageing mechanism of DC insulation under temperature gradient is investigated and at the same time the characteristic parameters during DC ageing is searched. Moreover, the method of status characterization for DC insulation is proposed and the life model of electrical-thermal (under temperature gradient) ageing for DC insulation is built too. The results will be expected to provide experimental and theoretical research basis for the condition monitoring and life evaluation of HVDC cables.

近年来，国内外高压直流电缆的发展十分迅速，其研发和投运量逐年增长。然而对于高压直流电缆的长期老化性能的研究仍很欠缺。目前，国内外学者普遍认同空间电荷在高压直流电缆的运行和老化过程中发挥着决定性的作用。为此已开发出具有抑制空间电荷特性的新型直流电缆绝缘材料，其中以纳米复合材料为代表。但是新型纳米复合材料的长期特性和寿命评价方法还有待研究。另外，以往研究表明，实际电缆运行中的温度梯度效应严重畸变了绝缘低温侧的场强分布。然而温度梯度下直流绝缘的长期老化破坏机理的研究仍属空白。 本项目通过研究温度梯度-直流电场协同作用下高压直流电缆料的各种宏-微观性能随老化过程的变化规律，探讨温度梯度场下直流绝缘的老化机理，寻找直流老化过程中的特征参量，提出直流绝缘状态表征方法和建立直流绝缘电-热（温度梯度场）老化寿命模型。研究结果有望为高压直流电缆的状态监测和寿命评估提供实验和理论研究依据。

电力设备中绝缘材料在运行过程中受到长期电热联合作用，内部结构持续发生变化，电气性能逐渐下降，尤其在高压直流电场作用下，空间电荷效应协同绝缘内部温度梯度效应，严重影响绝缘材料的长期使用寿命。.为研究实际工作条件下高压直流交联聚乙烯（XLPE）电缆的长期运行可靠性，采用自制多组片状试样在直流电场与温度梯度/高温作用下的联合老化装置分别对三种XLPE（国产XLPE、国产纳米改性XLPE、进口XLPE）进行长期直流电场-温度梯度联合作用下的加速老化，并分别测量了不同老化形式（高温和温度梯度）和不同老化时间（未老化、老化一个月、三个月以及半年）试样的空间电荷、直流电阻率、热氧老化状况、介电损耗、结晶度、活化能等参数随老化的变化情况，通过对比分析直流电场协同高温或温度梯度场下XLPE绝缘介电性能和微观特性的演变规律及异同点，查明纳米改性XLPE用于国产直流电缆绝缘料长期运行的可行性。.研究结果表明：三种XLPE老化过程中存在着电热降解与交联反应两个相互矛盾的作用。短期电热老化过程中的电热降解作用使得XLPE大分子链断裂生成小分子产物，试样的晶区结构遭到破坏，试样介电性能与热稳定性均有所降低，试样内部靠近电极侧出现异极性空间电荷积聚，并向试样内部引入浅陷阱。随着老化继续进行，老化小分子产物作为晶核引发交联反应，交联反应逐渐占据主导，晶区形态有所完善，介电性能仍有所下降，但热稳定性有所提升，开始出现同极性空间电荷积聚，内部浅陷阱继续增加。长期老化过程中，由于过度交联阻碍了重结晶过程，在电热联合作用下，结晶形态又发生破坏，热稳定性降低，深陷阱的增多使得测得的介电性能有所提升，同极性空间电荷积聚减少。对于老化相同时间的试样，温度梯度老化后试样的低温老化侧空间电荷积聚多于高温老化试样，极性和介电损耗高，电阻率低，去极化电流衰减较快，热氧老化严重，短期老化结晶形态也不如高温老化试样完善，热稳定性差；长期老化较为改善。表明温度梯度老化对材料的介电性能以及微观性能的破坏作用强于单纯的电热老化。另外，添加1wt%纳米SiO2的纳米改性XLPE材料，其长期老化后强场电导、空间电荷特性等介电性能以及热稳定性均优于纯XLPE。